The present invention relates to an automatic system for control of the mixture strength supplied in slow-running conditions to a heat engine having an electronic fuel injection system, in particular a sequential and phased system, and including a valve for supply of supplementary air in adjustable quantities, generally disposed to divide a duct connecting zones upstream and downstream of the butterfly valve controlled by the accelerator.
As is known, drift of the petrol/air mixture strength with which a heat engine is supplied is a rather typical phenomenon so much so that periodic adjustment has to be made to the supply system both in new systems and during its lifetime, following ageing of the engine and drift of its components. Such drift of the mixture strength is particularly unwanted in the case of electronic injection systems which due to their better operation necessitate very precise general control strategies of operation of the engine, in that there exists an electronic central control unit which, in dependence on signals which is receives from various sensors (principally sensors detecting the speed of rotation and phases of the engine, and sensors detecting the pressure and temperature of the inducted air) determines for example the density of the air in the manifold and the speed of rotation of the engine, from which, in dependence on the desired mixture strength it calculates through an interpolation on respective memorized mappings a phase and duration of injection of the fuel at the injectors as well as the ignition advance. Currently the operator effects periodic adjustment of the mixture strength by detecting the concentration of exhaust gas at slow running, by acting on a trimmer which corrects the duration of the injection time.